1. Field of the Invention
The present invention relates to a numerical controller, and in particular, to a numerical controller that dynamically switches a time constant for an acceleration and deceleration filter.
2. Description of the Related Art
A linear acceleration and deceleration control system is known as a system that controls acceleration and deceleration of a servo motor. In the linear acceleration and deceleration system (left of FIG. 15), the acceleration changes rapidly, thus disadvantageously shocking a servo control system and a loading system therefor, resulting in the likelihood of vibration. To avoid such a disadvantage, a bell-shaped acceleration and deceleration system (right of FIG. 15) has been used in which a tangential jerk is suppressed by applying a time constant filter to the linear acceleration and deceleration. In the bell-shaped acceleration and deceleration, the acceleration and deceleration have values expressed in Equation (1) as follows.
                    Jerk        =                  Acceleration                      Time            ⁢                                                  ⁢            Constant                                              (        1        )            
As indicated by Equation (1), to set a jerk equal to or smaller than a predetermined limit value in the bell-shaped acceleration and deceleration, the time constant needs to have a large value. However, an increased time constant reduces the jerk, while increasing a cycle time. Thus, operators desire to use the minimal time constant required to the extent that machines are prevented from being significantly shocked.
As a related art that controls the speed, acceleration, and jerk on axes provided in a machine during driving, for example, International Publication WO 2006/063945 discloses a technique in which a tangential limit speed, a tangential limit acceleration, and a tangential limit jerk are provided for each section of a path, and speed curves are created which extend in opposite directions from a section with a locally minimum limit speed and are connected together to create a speed curve that meets the limit.
Japanese Patent Application Laid-open No. 06-095720 discloses a technique in which, when a machining program for a numerical controller is executed in which discrete time constants are provided for a plurality of axes, speed control is performed by using, for an operation using only a single axis, the time constant for that axis and using, when a plurality of axes are simultaneously operated, the largest one of the time constants for the axes.
In a driving system operated using a plurality of axes, the ratio between the jerk and the acceleration may vary among the axes due to, for instance, a variation in rigidity among components used for the axes. In such a case, the minimal time constant required varies among the axes used to operate the driving system (the minimal time constant required varies according to a travel direction), but the conventional numerical controller fails to change the time constant during machining and constantly maintains the time constant independently of the axis when a time constant filter is applied. Thus, when a driving system operated using a plurality of axes are controlled, the maximum value of the time constant needs to be used which results from calculation for all the axes that may be controlled. Thus, a time constant larger than the minimal time constant required may be used according to the travel direction, disadvantageously increasing the cycle time.
For example, as depicted in FIG. 16, in a driving system operated on two axes of an X axis and a Y axis, the same acceleration limit is assumed to be used both for the X axis and for the Y axis, and a jerk limit for the Y axis is assumed to be twice as high as a jerk limit for the X axis. In such a case, the minimal time constant required for traveling in the direction of the X axis has twice as large a value as the minimal time constant required for traveling in the direction of the Y axis. However, since the time constant is inhibited from being changed during operation, the time constant is set to a value (larger value) that meets the acceleration limit and the jerk limit for the X axis. Thus, a sufficient jerk fails to be obtained in the direction of the Y axis, resulting in an increased cycle time.
When the technique disclosed in WO 2006/063945 is applied in order to solve the above-described problem, the jerk needs to be directly controlled so as to have an appropriate value within the range of limitation. In that case, a complicated path through which a control target is moved increases the amount of calculation that needs to be processed per unit time, leading to the need for a high-performance CPU and the like. This poses a problem with costs.
Even when the technique disclosed in Japanese Patent Application Laid-open No. 06-095720 is applied, the technique fails to switch the time constant during operation. This disadvantageously precludes the optimal time constant from being continuously used when the travel direction changes.